Free fatty acid (FFA) can directly affect insulin release and indirectly affect insulin sensitivity. In search for novel G-protein coupled receptors (GPCRs), four FFA receptors were discovered, sequenced and de-orphanized. GPR41 was discovered in 1997 and initially cloned from rat lung while the human GPR41 sequence was published later the same year as part of an effort to identify galanin GPCR subtypes (Sawzdargo, M., et al., Biochemical and Biophysical Research Communications, 1997. 239(2): p. 543-7). Four GPCRs were identified (GPR40, GPR41, GPR42 and GPR43) and found as a cluster near chromosome 19q13.1. The receptors fall within the same GPCR subfamily but do not share a great deal of homology and differ in their tissue distributions and ligand specificities. While GPR40, GPR41 and GPR43 are functional receptors activated by various FFAs, GPR42 is non-functional and is thought to be the product of a polymorphic gene insert (Brown, A. J., et al., Journal of Biological Chemistry, 2003. 278(13): p. 11312-11319).
GPR40 is specifically activated by medium to long chain FFAs and signals through Gq protein coupling (Briscoe, C. P., et al., Journal of Biological Chemistry, 2003. 278(13): p. 11303-11311; Kotarsky, K., et al., Biochemical and Biophysical Research Communications, 2003. 301(2): p. 406-410). The tissue distribution as determined by rtPCR (reverse transcription Polymerase Chain Reaction) found that pancreas and brain expressed far more GPR40 than any other tissue (Briscoe, C. P., et al., Journal of Biological Chemistry, 2003. 278(13): p. 11303-11311; Kotarsky, K., et al., Biochemical and Biophysical Research Communications, 2003. 301(2): p. 406-410; Itoh, Y., et al., Nature, 2003. 422: p. 173-176). No expression of GPR40 was found in human peripheral blood mononucleocytes, B-lymphocytes or neutrophils (Briscoe, C. P., et al., Journal of Biological Chemistry, 2003. 278(13): p. 11303-11311). Within the pancreas, the expression was localized to the islets. Double staining with in situ hybridization in rat pancreas found GPR40 expression in those cells staining for insulin (β-cells) but not glucagon (α cells) (Itoh, Y., et al., Nature, 2003. 422: p. 173-176). Further supporting the islet specificity of GPR40 is the fact that both murine MIN6 and rat INS-1E pancreatic β-cell lines express GPR40 homologues and display an intracellular calcium response to long chain FFAs (Briscoe, C. P., et al., Journal of Biological Chemistry, 2003. 278(13): p. 11303-11311; Kotarsky, K., et al., Biochemical and Biophysical Research Communications, 2003. 301(2): p. 406-410). Long chain FFAs also stimulate insulin secretion from MIN6 cells in vitro. Insulin secretion is greater in the presence of high glucose indicating that FFAs amplify glucose-stimulated insulin release. To relate the expression of GPR40 to an in vivo state, the expression of the murine GPR40 homologue was compared in the pancreas of obese ob/ob mice and lean mice. In whole pancreas GPR40 was 6.5 fold greater in the ob/ob mice.
Adipose tissue plays an active role in energy balance far beyond that of simple energy storage. Elevated blood levels of FFA are common in obese and diabetic patients and have been implicated in insulin resistance and reduced glucose uptake. There is an enormous body of evidence relating FFAs to obesity, the metabolic syndrome and diabetes (see, e.g., Boden, G., Diabetes, 1997. 46(1): p. 3-10; Arner, P., Trends in Endocrinology and Metabolism, 2003. 14(3): p. 137-145). There is a continuing need for new GPR40 agonists, which can activate the pancreatic GPR40 receptor and stimulate glucose-induced insulin secretion. There is a further need for new GPR40 agonists that are efficacious in both lean and obese subjects. There is a further need for new GPR40 agonists for the treatment of insulin resistance, hyperglycemia, obesity, diabetes such as Non-insulin Dependent Diabetes Mellitus (NIDDM), and other disorders related to lipid metabolism, energy homeostasis, and complications thereof.